Various proposals have been advanced to provide a vehicle tire with bullet-proof capabilities. In one such proposal an elastomeric reslient foam is formed in situ within a conventional tire carcass after the carcass has been mounted on a wheel. The elestomeric foam is formed to have a substantial number of closed cells, so that a bullet, fragment or projectile can pass through the carcass and foam without rupturing any cells other than those directly in the projectile's trajectory. The intent is that the tire will be able to continue to support the vehicle, using the undamaged cells for support purposes.
The foams used in foam-filled tires are preferably internally pressurized, since conventional non-pressurized or open-celled foam materials are unduly compressed by externally applied loads. For example, conventional open-celled flexible foams can be compressed to at least one-half of their original dimensions by the application of load forces in the neighborhood of 2 p.s.i. In general, the compression characteristics of conventional open-celled foam materials tend to make them unsuitable as the sole resilient filling material in tire carcasses, since such foams would tend to remain in the compressed state under the relatively high loads imposed by most vehicles.
To alleviate the deficiencies of conventional foams, it has been proposed to internally pressurize the cells by suitably regulating the conditions of the foam-forming process; see for example U.S. Pat. No. 3,022,810 issued to D. M. Lambe on Feb. 27, 1962.
In the process of forming a resilient flexible foam, the polymerization or vulcanization reaction and the gas-forming reaction proceed simultaneously. The character of the product is determined at least partly by the relative rates of the two reactions. As polymerization or vulcanization proceeds the viscosity of the liquid or unvulcanized solid phase changes so as to influence its surface tension. The surface tension largely determines the resistance to bubble development. Gas release can be accomplished when the gaseous reaction product quantitatively exceeds the point of supersaturation in the liquid or unvulcanized solid phase; the gas then comes out of solution to deform the liquid or solid phase into the form of tiny bubbles, similar to the commonly seen soap bubbles produced by blowing air through a pipe. The tiny bubbles propagate into larger bubbles or cells against the resistance offered by the phase that forms the cell walls. As explained by Calvin J. Benning in his book, "PLASTIC FOAMS: THE PHYSICS AND CHEMISTRY OF PRODUCT PERFORMANCE AND PROCESS TECHNOLOGY, VOL 1 -- CHEMISTRY AND PHYSICS OF FOAM FORMATION" published by John Wiley & Sons in 1969, at page 130, at least some of the smaller cells rupture to form larger cells and/or to combine with the larger cells.
The permissible radius of a cell is said to be directly proportional to the surface tension of the liquid or unvulcanized solid phase and inversely proportional to the pressure difference across the membrane cell wall. Therefore, with a given surface tension the development of an increasing internal gaseous pressure will cause the bubble to burst when the cell reaches some predetermined radius or size. If such bursting is not controlled the formed cells may contain an excessive number of open cells, which is undesired in a bullet-proof tire.
Under conventional practice, if the cells are to remain internally pressurized, as desired for a foam-filled tire, the cell radius must be limited to a small value; otherwise the cells will burst and become depressurized. The small size cells must have relatively thick walls to contain the internal gaseous pressure (i.e. thick in relation to the cell volume). These considerations have conventionally led to the use of high density foams. Such high density foams impose a weight penalty on the vehicle, as well as a sacrifice in riding quality. The vehicle load is transmitted to a large extent through the cell walls rather than through pressure variations of the contained gas.
In conventional foam-forming processes the cell walls must be of sufficient strength,, while in the viscous pre-cured state, to contain or resist the internal gas pressures. Unfortunately the cell wall material does not possess its highest potential strength while in this state; i.e. the highest strength of the cell wall is not attained until after polymerization or vulcanization and cure of the final foam. The lack of strength possessed by the cell wall during bubble development means that the blowing pressure must be kept low. The end result is that the cell wall, rather than the entrapped gas, must carry the major part of any load imposed on the tire. This means that more foam material is required than would be the case if the foam was developed so that the entrapped gas carried the major part of the load.
As previously noted, the employment of large quantities of foam-filling elastomers in a tire is disadvantageous because the foam adds to the weight of the tire and therefore adds to the dead load that must be moved by the power source. For example, one tire carcass weighed approximately 27 pounds empty and 90 pounds when filled with foam.
Large masses of foam-forming elastomers are also undesirable since they contribute to increased rolling resistance, hence to heat build-up and potential degradation of the foam. Additionally, such large foamed elastomeric masses tend to increase raw material costs and curing time. Large foamed elastomer masses may also tend to increase the centrifugally imposed stresses on the tire carcass, thereby tending toward greater wheel imbalance and shortened tire life.